(a) Field of the Invention:
The present invention concerns an improvement in photoconductive photoreceptots used in, for example, electrophotography, and more particularly it relates to an improvement in photoreceptive materials consisting mainly of amorphous silicon.
(b) Description of the prior art:
In the past, as the photoconductive photoreceptive materials employed in, for example, electrophotographic printers and laser printers, there have been used, in general, such substances as selenium (Se), cadmium sulfide (CdS), zinc oxide (ZnO) and organic photoconductive materials. Recently, there has been developed a method of using in place of those materials mentioned above, amorphous (noncrystalline) silicon (hereinafter referred to as a-Si). With respect to the basic composition of a-Si, there is made a detailed description thereof in U.S. patent application Ser. No. 342,650 now U.S. Pat. No. 4,532,196, and its manufacturing conditions are described in detail in U.S. patent application Ser. No. 372,826.
As compared with the known photoreceptive materials mentioned above, the photoreceptive material made of a-Si has the advantages represented by its excellent holdability of electric charge and sensitivity to light, as well as the ability of presenting clear images, in addition to the superior thermal strength, chemical stability and mechanical strength, and furthermore its harmlessness to human body and low manufacturing cost.
Such a-Si photoreceptor is obtained by forming, on top of a conductive substrate by relying on such method as plasma CVD technique, sputtering technique and evaporation-deposition technique. However, in case, for example, an a-Si film is formed by glow discharge using a high frequency in a gas atomosphere containing, for example, silane gas, the substrate requires to be held for a certain length of time at a temperature ranging from 200.degree. to 300.degree. C. The a-Si layer thus formed usually contains therein large distortions. In case, for example, the electroconductive substrate is a cylindrical drum, it will develop residual compressive stress therein, which can cause deformation or injury of the substrate per se.
Also, in case the a-Si photoreceptor thus formed is used to perform printing, the first step is to subject this photoreceptor to corona discharge under dark condition to charge this photoreceptor, and thereafter the image of an object such as a picture, a letter or a symbol which requires to be transferred is formed on this photoreceptor by an appropriately designed optical system. Since this photoreceptor has photoconductivity, the electric charges which this photoreceptor have been carrying in correspondence to the various levels of brightness of light coming from various portions of the object are discharged, and as a result, an electrostatic latent image of the object is formed on the photoreceptor. This electrostatic latent image is then caused to electrically attract the fine printing particles. And, these fine particles of the electrostatic latent image are deposited on a transfer sheet and are fixed thereon. Thus, a transfer of the image of the object is performed. During the course of formation of this electrostatic latent image, there takes place a migration of carriers between the a-Si layer and the electroconductive substrate. This migration greatly affects the characteristic of the photoreceptor due to, for example, the condition at the interface between the a-Si layer and the substrate or the type of material of the substrate.
Accordingly, it is needless to say that, for the substrate of an a-Si photoreceptor, there ought to be used such a material as will not deteriorate the characteristic of the photoreceptor, and besides, in view of the temperature condition required at the time of formation of the a-Si film, there cannot be employed a substrate having a low melting point. Furthermore, a substrate having such a low mechanical strength as will develop deformation due to interal stresses within the a-Si film is not desirable.
As discussed above, in the selection of the conductive substrate, it is required to use such a material as is superior in both thermal and mechanical strength, as well as satisfies the requirement of durability and the ability to sufficiently maintain the characteristic as a photoreceptor.
In spite of the fact that research and development of photoreseptors per se are presently making a progress in general, there has not been found or established a uniform view as yet with respect to the electroconductive substrate supporting such researches, especially with respect to the material of the substrate.